1. Field of the Invention
This invention relates to method and apparatus for forming fibrous webs from a gaseous suspension of fibers, and more specifically to an improvement in such method and apparatus for achieving formation of fibrous webs having high quality edges.
2. Description of the Prior Art
Air-laid fibrous webs are becoming extremely popular for single, or limited use products, such as disposable diapers, sanitary napkins, cosmetic pads, industrial towels, household towels, tissues and the like.
A commonly employed apparatus for forming an air-laid fibrous web includes a fiberizing roll for separating fibers from a fibrous feed mat and entraining the fibers in air to form an air suspension of said fibers. The air suspension of fibers is directed through an upstream end of a formation duct toward a moving foraminous forming surface which traverses a downstream open end of said duct. The air from the suspension is directed through the foraminous forming surface, and the fibers from the suspension are condensed upon the surface to form the air-laid fibrous web. A partial vacuum is established behind the forming surface to aid in directing the air suspension of fibers toward it. Quite often the formed webs are subjected to conventional post-treatment operations; such as embossing, adhesive bonding and the like; to impart desired properties to the webs.
Uniformity of web formation is enhanced by providing substantially uniform flow of the gaseous suspension of fibers from the fiberizing roll to the forming surface. Uniform flow is enhanced by establishing a low depth ratio between the upstream end of the formation duct and the location at which fibers are diverted from the fiberizing roll toward said formation duct in a direction which is substantially tangent to the outer periphery of said fiberizing roll. This latter location is approximately where the outer periphery of the fiberizing roll most closely approaches an extended section of the bottom wall of the formation duct. The above-described depth ratio will hereinafter be referred to as "D.R.".
Although the uniformity in basis weight of the major portion of the web has been improved by reducing the D.R., the longitudinal edges of the web have been unacceptably nonuniform in basis weight. Accordingly, it has been necessary to sever these edges from the main body of the web in order to produce acceptable nonwoven products from said web. Although the removed edges can be reprocessed, the required removal of these edges renders the web forming operation less efficient than would be the case if the edges were sufficiently uniform in basis weight so that they could be retained in the web during subsequent processing of said web into nonwoven products.
U.S. Pat. No. 3,797,074, issued to Zafiroglu, discloses an air-laydown apparatus and method in which a partial vacuum is established through a lower duct surface upstream of a fiberizing roll for removing any turbulent boundary layer which might develop. To achieve this objective the vacuum is established over the entire cross-machine-direction dimension of the duct. The arrangement disclosed in the Zafiroglu patent is not effective in removing standing vortices in a fiber laden gas stream downstream of a fiberizing roll created by the high speed rotation of said fiberizing roll. In fact, the Zafiroglu patent is concerned with an entirely different problem than that which applicant has discovered and solved. Specifically, Zafiroglu is concerned with the removal of low velocity air flow which occurs at air-confining boundaries over which the flow is directed.